The equivalent electrical circuits of two prior art resistive loads are shown in FIGS. 1 and 2. FIG. 1 shows an asymmetrical configuration of cells while FIG. 2 shows a symmetrical configuration. They comprise lumped series resistance values R1 in both cases and lumped parallel resistance values R2 in the asymmetrical case and 2R2 in the symmetrical case. In either case the iterative impedance of each cell is equal to .sqroot.R1R2 and the attenuation is proportional to R1 and inversely proportional to R2. At hyperfrequencies it is the practice to use microstrips for making distributed-constant resistances. FIG. 3 shows a resistive strip of width W deposited on one face of a dielectric substrate whose other face is covered in a layer of conductive metal. The dielectric has a thickness h and a relative dielectric constant .epsilon.. In this embodiment the resistance of the resistive layer per unit area is proportinal to the area. The resistive layer may be made from series 1610 material sold by Dupont and Nemours and which can be made to have a resistance of 10 ohms to one megohm for a standard sample which is 5 mm long by 2.5 mm wide by 25 micrometers thick (before baking). The characteristic impedance of the attenuation circuit is proportional to the logarithm of the ratio of the dielectric thickness h by the width W of the strip, and inversely proportional to the square root of the relative permitivity .epsilon.. Thus between the attenuator's input E and its output S there is a distributed-constant series resistance R1 sandwiched between two distributed-constant parallel resistances 2R2. Up till now, the resistance R1 has been rectangular in shape and of low resistivity, while the resistances 2R2 have been likewise rectangular in shape but of very high resistivity, at least for low attenuation. Two return conductors are placed on the top face of the substrate to make a connection over the edge of the substrate to the metal layer on the other face.
The FIG. 3 prior art configuration gives rise to a constant characteristic impedance and a constant coefficient of attenuation per unit length since the series resistance R1 and the parallel resistances 2R2 are themselves uniform. As a result of the attenuation per unit length being constant from the input E to the output S, the power dissipated in successive sections of equal length along the attenuator is far from equal in a conventional attentuator and decreases from a maximum at the input to a minimum at the output. Supposing that the attenuator is divided into equal sections 1 to n each having an attenuation coefficient k, the power dissipated in the n-th section, Pd.sub.n is given by the equation: ##EQU1## where P.sub.O is the input power.
The drawback of such a technique is that a hot point is created at the input to the attenuator, thereby limiting the maximum power which it can dissipate, long before the remainder of the attenuator is in danger of getting hot. Thus inefficient use is made of the substrate area.
Preferred embodiments of the present invention mitigate this drawback by spreading power dissipation more evenly over the available substrate area, thereby enabling more power to be dissipated for a given area of substrate.